Mechanism of mediating macrophage- hepatic stellate cell crosstalk during liver fibrosis through chemokine receptor CCR9

Chemokine receptors mediate migration of immune cells into the liver, thereby promoting liver inflammation. Crosstalk between immune cells and hepatic stellate cells (HSCs) is known to be important step toward liver fibrosis. Accumulation of TNF-α-producing CCR9+ macrophages was noticed during the process of liver fibrosis. Increased CCR9 expression was also found on activated HSCs. With dual-color immunofluorescence staining, we confirmed that CCR9+ HSCs co-localized with accumulated CCR9+ macrophages around periportal areas during liver fibrosis. Importantly, experimental liver fibrosis was significantly ameliorated in CCR9-/- mice compared with wild-type (WT) mice, assessed by α-SMA immunostain, sirius red staining and mRNA expression levels of α-SMA, collagen 1α1, TGF-β1, and TIMP-1. Hepatic CD11b+ macrophages from CCl4-treated WT mice (i.e. CCR9+ macrophages), but not non-CD11b+ cells, significantly activated HSCs in vitro compared with those from CCR9-/- mice. TNF-α or TGF-β1 antagonism attenuated CCR9+ macrophage-induced HSC activation. Furthermore, addition of retinoic acid up-regulated CCR9 expression on intrahepatic CD11b+ macrophages from WT mice in vitro, but not on those cells from the spleen. Interactions between macrophages and HSCs via retinoic acid may indicate a highly privileged environment for CCR9 in the promotion of liver fibrosis. In conclusion, accumulated CD11b+ macrophages are critical for activating HSCs through the CCR9 axis and therefore promote liver fibrosis. CCR9 antagonism might be a novel therapeutic target for liver fibrosis.